Electrode for a high-pressure discharge lamp



6, 1969 J. F. SMALLEY 3,463,958

ELECTRODE FOR A HIGH-PRESSURE DISCHARGE LAMP Filed Sept. 12, 1967 Fig.2.

United States Patent 3,463,958 ELECTRODE FOR A HIGH-PRESSURE DISCHARGELAMP John F. Smalley, London, England, assignor to British LightingIndustries Limited, London, England Filed Sept. 12, 1967, Ser. No.667,273

Claims priority, application Great Britain, Sept. 13, 1966,

40,832/66 Int. Cl. H01j 17/04, 61/06 U.S. Cl. 313211 3 Claims ABSTRACTOF THE DISCLOSURE Electrodes for a high pressure mercury 'vapourdischarge lamp consist of a tungsten rod about which a layer or layersof a tubular braid of tungsten wires are wound, the braid being filledwith thermionically emissive material.

The invention relates to an electrode for a high pressure discharge lampand in particular, to an electrode for use in a high pressure mercuryvapour discharge lamp.

Electrodes of high pressure mercury vapour lamps usually consist of arod of a refractory metal such as tungsten containing a small percentageof thorium or thoria, about which are placed two concentric spirals oftungsten wire which have been coiled in such a way that the inner spiralfits closely on the central tungsten rod and the outer spiral fitsclosely On the inner spiral. The inner spiral may consist of a number ofturns of tungsten wire with predetermined spaces between each turn. Theouter spiral may consist of an equal number of more turns than the innerspiral, the turns may have a predetermined space between them. The sizeof the central rod and the diameter of wire for the inner and outerspirals of tungsten and the total length of the spirals is governed bythe wattage of the lamp into which they are to be sea-led. During thepreparation of the electrodes, the spirals are generally coated with apaste of alkaline earth carbonates such as barium carbonate. Thecarbonates are subsequently reduced to oxides having a low work functionand thus providing an electron-emitter on the electrodes. The amount ofemissive material and its adhesion to the electrodes is governed by thespacing between the turns of wire in the tungsten spirals. Any surplusemissive material which may collect on the outer spiral has to beremoved before the electrodes are fitted into the lamps or it has to beheated so that it adheres to the surface of the spiral. In suchelectrodes the amount of emitter is limited to the amount which can bemade to adhere to the tungsten spirals. A disadvantage of theseelectrodes is that the emitter may be sputtered from the spirals whenthe lamp is operated over many hours, collecting on the arc tube innersurface and eventually reducing the light output of the arc tube.

According to the invention there is provided a electrode for a highpressure discharge lamp consisting of a tubular braid of tungsten wireswhich is filled with thermionically emissive material and is coiledabout a tungsten rod.

An embodiment of the invention will now be described with reference tothe accompanying drawing, in which,

FIGURE 1 is a partially cut-away view of an electrode constructedaccording to the invention, and

FIGURE 2 is a view of an arc tube of a discharge lamp incorporatingelectrodes as shown in FIGURE 1.

3,463,958 Patented Aug. 26, 1969 ice Electrodes 10 project from thepinch-seal ends 11 of a quartz arc tube 12 of a high pressure mercuryvapour discharge lamp as shown in FIGURE 2. The ends of the electrodeswithin the seal are welded to a molybdenum foil 13 which has a thicknessof 0.0014 inch and a width of 4 mm. Lead-in wires 14 are welded to thefoils.

As shown in FIGURE 1 the electrodes 10- consist of a central tungstenrod 15, containing 0.75% of thoria, which is surrounded by a doublehelical coil 16, held in position by friction. The coil is formed from anumber of tungsten wires which have been braided together in the mannerde scribed in British patent specification No. 931,059. The braid isfilled with oxides of alkaline earths 17 such as barium oxide. Theseoxides are put in the braid during manufacture in the form of a paste ofcarbonates which is substantially reduced by heat, the end of the coilis spaced from the tip 18 of the central rod. An auxiliary electrode 19is provided at one end of the lamp being connected to a lead wire 20through a molybdenum foil seal 21.

The lamp is filled with mercury vapour and argon and may contain theiodides of sodium, scandium, thorium, indium, thallium and gallium. Onstarting the lamp, a glow discharge is initiated between the coils ofthe two electrodes 10 with the aid of the auxiliary electrodes 19. Theglow discharge is provided by the oxides held within the coils which areelectron-emissive substances. The glow discharge rapidly transforms intoan arc discharge between the tips of the electrodes.

The fact that the emitter is held within a tube of braided wires greatlyreduces the sputter of emitter during the period of the glow discharge.Further the fact that the tube is hollow enables more emitter to be heldin each electrode than is possible with a solid wire overWind. Thisincreases lamp life and luminous performance through life. The finewires which form the braid also improve field emission of the electrodeso that lamp starting is better. In addition, because of the low thermalmass on the outer surface of the braid, the transitio from a glowdischarge to an arc discharge takes place in a shorter period of time.

What is claimed is:

1. In a high pressure discharge lamp, an electrode consisting of atungsten rod, a tubular braid of tungsten wires coiled about said rod toform a coil thereon of several turns close together, and a filling ofthermionically emissive material withi the tubular braid.

2. An electrode as claimed in claim 1, wherein the braid is coiled aboutthe rod in two layers, each layer forming a coil on said rod of severalturns close together.

3. In a high pressure discharge lamp as claimed in claim 1 a secondelectrode constructed in a similar manner to the first electrode, and anauxiliary electrode.

References Cited UNITED STATES PATENTS 6/1965 Speros 313346 X 5/1966Antonis et a1. 313-346 U.S. Cl. X.R.

